1. Field of the Invention
This invention relates to an integrated semiconductor light-emitting device which emits a plurality of laser lights of different wavelengths, and a method for manufacturing the integrated semiconductor light-emitting device.
The present application claims priority from Japanese Application No. 2004-16023, the disclosure of which is incorporated herein by reference.
2. Description of the Related Art
These days, mass digital contents have become easily available because of proliferation of a digital broadcast and broadband, and so there is a need for higher density information processing technology.
For example, in the case of an optical disc used for recording/reproducing in an information recording/reproducing system such as a CD player and a DVD player, an increase in density from a CD (Compact Disc) with a capacity of 700 MB using a laser light with a wavelength of 780 nm band to a DVD (Digital Versatile Disc) with a capacity of 4.7 GB using a laser light with a wavelength of 650 nm band has been progressively attained. A further higher density optical disc with a capacity of 20 GB or more using a laser light with a wavelength of 405 nm band has been realized in recent years.
However, the information recording/reproducing system capable of recording on and reproducing from a high density optical disc needs a pickup equipped with an integrated semiconductor light-emitting device which is capable of emitting laser lights with wavelengths of 650 nm band and 780 nm band in addition to a laser light with a wavelength of 405 nm band, to provide compatibility enabling a user to continuously use the various optical discs which have been accumulated up to this time.
For reduction in size, weight and the like, it is desirable for a pickup having compatibility with a DVD to be equipped with the integrated semiconductor light-emitting devices for emitting two laser lights with wavelengths of 405 nm band and 650 nm band. However, it is impossible to realize such integrated semiconductor light-emitting devices on the same substrate by the use of monolithic semiconductor manufacturing technology. For this reason, an integrated semiconductor light-emitting device of a hybrid structure (2-wavelength laser device) is suggested by Japanese unexamined patent publication 2002-118331, for example.
As disclosed in FIG. 1 of the publication 2002-118331, the integrated semiconductor light-emitting device includes a 405 nm band InGaAlN-based semiconductor laser (i.e. first semiconductor light-emitting device) LD1 and a 650 nm band InGaAlP-based semiconductor laser (i.e. second semiconductor light-emitting device) LD2. The first and second semiconductor light-emitting devices LD1 and LD2 are combined by the use of a direct bonding (wafer fusion) technique to bond together a p-type GaN contact layer 101 and a p-type GaAs contact layer 121 which are respectively formed on the first and second semiconductor light-emitting devices LD1 and LD2.
When a drive current is fed through a common p-electrode 131 joined to the contact layers 101 and 121, the drive current flows through the contact layers 101, 121 and cap layers 102 and 122 and so on to active layers 107 and 126 for generation of laser light.
For manufacturing the conventional integrated semiconductor light-emitting device, the first things to be prepared are a first laser wafer for producing a plurality of first semiconductor light-emitting devices LD1 and a second laser wafer for producing a plurality of second semiconductor light-emitting devices LD2. The first and second laser wafers are then bonded by the direct bonding technique, and then cleaved into chips. For the use of the direct bonding technique to manufacture integrated semiconductor light-emitting devices having uniform qualities in terms of optical properties and the like, it is required that the surfaces on the side to be bonded in the first and second laser wafers have high degree of flatness.
This limitation makes it impossible to obtain an integrated semiconductor light-emitting device because of the difficulty of using the conventional direct bonding technique to bond a laser wafer for producing the semiconductor light-emitting devices having an uneven bonding surface to another laser wafer in a construction thereof.
For example, the first semiconductor light-emitting device LD1 used in the conventional integrated semiconductor light-emitting device is formed of the so-called buried-type GaN laser device having n-type InGaAlN current confinement layers 103 provided on both sides of a stripe-shaped p-type AlGaN cladding layer 104 and a stripe-shaped p-type GaN cap layer 102. However, a typical GaN laser device emitting a short-wavelength laser light (e.g. wavelength of 405 nm band) rarely has the buried structure, and often has ridge stripe structure capable of providing performance superior to that in the buried structure.
If the GaN ridge-stripe laser device is used instead of the first semiconductor light-emitting device LD1 with the buried structure, the surface of the semiconductor is covered with an SiO2 insulating film and a ridge portion projects, for example. Hence, the bonding surface results in surface asperity, which in turn makes it impossible to use the conventional direct bonding techniques to bond the first and second semiconductor light-emitting devices LD1 and LD2 together.
In consequence, an integrated semiconductor light-emitting device using the GaN ridge-stripe laser is not able to be realized by the use of the conventional direct bonding techniques.
In the conventional integrated semiconductor light-emitting devices, a p-electrode is common to both of the first and second semiconductor light-emitting devices LD1 and LD2, so that electric current flows through the p-type GaN contact layer 101 and the p-type GaAs contact layer 121 to the respective stripe-shaped cap layers 102 and 122 from the in-plane direction (lateral direction).
However, both the p-type GaN contact layer 101 and the p-type GaAs contact layer 121 which are the semiconductor layers, have a far less than sufficient degree of electric conductivity, and therefore are of high resistance for serving as the path for an electric current inflow in the lateral direction. Thus, an increase in drive voltage and an increase in electrical power consumption are caused, leading to various problems such as the need to provide a large-sized heat dissipating structure.
Further, in the conventional integrated semiconductor light-emitting devices, emission spots of the first and second semiconductor light-emitting devices LD1 and LD2 are arranged in the vertical direction with respect to the active layers 107 and 126 (i.e. the thickness direction of the actice layers 107 and 126). If the integrated semiconductor light-emitting device having such positional relation between these two emission spots is mounted on the pickup, the following problen arises.
As schematically illustrated in FIG. 10A, the pickup uses an objective lens OBJ to concentrate two different wavelength laser lights S1 and S2 on a signal recording layer of an optical disc DSC. The portion indicated with the reference symbol LD1 in FIG. 10A corresponds to the laser part for high density optical discs, which emits a blue laser light with a wavelength of 405 nm band. The portion indicated with the reference symbol LD2 corresponds to the laser part for DVDs which emits a red laser light with a wavelength of 650 nm band.
In this case, if the optical disc DSC is a DVD, the laser part LD2 is driven to emit the 650 nm laser light S2. The laser light S2 emitted from the laser part LD2 is focused on the signal recording layer of the optical disc DSC as a focused spot SP2 by the objective lens OBJ.
If the optical disc DSC is a high density optical disc, the laser part LD1 is driven to emit the 405 nm laser light S1. The laser light S1 emitted from the laser part LD1 is focused on the signal recording layer of the optical disc DSC as a focused spot SP1 by the objective lens OBJ.
Therefore, the focused spots SP1, SP2 on the signal recording layer of the optical disc DSC are affected by a shape of near field patterns (NFP) N1, N2 respectively emitted from the laser parts LD1, LD2.
Typically, the near field pattern of a semiconductor laser has an oval shape with a short diameter in a vertical direction y (the thickness direction of the active layer 107) with respect to the active layers 107, 126 and a long diameter in a horizontal direction x (the width direction) with respect to the active layers 107, 126 as indicated by the reference symbols N1 and N2 in FIG. 10A.
When the laser lights emitted from the oval-shaped near field patterns N1 and N2 are focused onto the optical disc DSC by the objective lens OBJ, because of the effect of the shape of the near field patterns N1 and N2, the focused spots SP1 and SP2 on the optical disc will be an oval shape with a short diameter in the vertical direction y and a long diameter in the horizontal direction x.
In the case that the optical disc DSC is a DVD, the integrated semiconductor light-emitting device is aligned so as to align the short diameter of the oval-shaped focused spot SP2 on the signal recording layer of the optical disc DSC with a tangential direction t to a track of the optical disc DSC as shown in FIG. 10B.
As compared with the case in which the integrated semiconductor light-emitting device is aligned so as to align the long diameter of the focused spot SP2 with the tangential direction to a track of the optical disc DSC, the alignment previously described substantially decreases the diameter of focused spot in the track direction of the optical disc DSC. Therefore, the signal resolution in the track direction is increased, so that high density recording becomes possible.
For this reason, the conventional integrated semiconductor light-emitting device needs to be mounted in the pickup so as to align the vertical direction y thereof with the tangential direction t with respect to the optical disc DSC as illustrated in FIG. 10B.
In this connection, suppose the optical disc DSC is a high-density disc and the optical system of the pickup is designed in such a way as to align the emission spot of the laser part LD1 which emits a 405 nm blue laser light with the optical axis center Q of the objective lens, the emission spot of the laser part LD2 which emits a 650 nm red laser light will be positioned out of the optical axis center Q of the objective lens.
When the optical disc DSC is changed to a DVD and the laser part LD2 emits a laser light, the laser light S2 does not have an incidence onto the signal recording layer of the optical disc DSC at a right angle to the signal recording layer. In other words, the laser light S2 enters the signal recording layer of the optical disc DSC at an inclined angle with respect to the tangential direction t to the track. The comatic aberration thus caused with respect to the tangential direction t results in a deterioration of the quality of the optical spot.
A typical pickup is equipped with a tilt correction mechanism for correction of an angle deviation between the disc surface and the optical axis of the objective lens which is caused by optical disc warp or the like. The tilt correction mechanism adjusts the angle of the pickup itself in order to keep an incident angle of the laser light on the signal recording layer of the optical disc at a right angle thereto at all times.
However, the tilt correction mechanism is capable of making an angle adjustment only to the tilt in the radial direction r of the optical disc DCS, and has no angle adjustment mechanism for a tilt in the tangential direction t. For this reason, it is impossible to make a tilt correction to the light coming in at an angle with respect to the tangential direction t which is caused when the integrated semiconductor light-emitting device is located as described above.
In consequence, if the conventional integrated semiconductor light-emitting device is mounted on a regular pickup, a correction to the incident angle of the laser light S2 entering at an inclined angle with respect to the tangential direction t is impossible. This results in further problem that the comatic aberration can not be reduced.